Thermosetting compositions, such as phenolic resins, have been known for many years and have been used in many applications because they have high heat resistance; they are light weight; they have excellent dimensional stability and they have very low flammability. The usual method for preparing thermoset molding compounds involves blending the resin with fillers, pigments and other additives, followed by compounding and granulating. The granulated resin compositions are then fabricated by any of the known methods such as compression, transfer, or injection molding.
Recently, there have been developed new fabrication methods in which liquid thermosetting resin compositions are injected directly into a mold where curing takes place, resulting in the formation of a fabricated part. Depending on the process steps and the nature of the compositions, these methods are known as liquid injection molding (LIM), reaction injection molding (RIM), or resin transfer molding (RTM). Liquid thermosetting resins also may be employed in sheet-molding compound (SMC) or pultrusion (PT) processes.
The liquid injection molding processes are fast and adaptable to high speed production requirements, and since the curing reaction is generally exothermic, these processes are less energy intensive than the usual methods for processing thermosetting resins. In addition, liquid injection offers the potential of precise orientation of reinforcing fibers in the mold prior to injection of the resin. This allows the production of parts with maximized strength to achieve engineering objectives. However, attempts to use these processes in the fabrication of products using conventional phenolic resins have not been completely successful because of the excess shrinkage of low solids content resins, and the residual unbound water and volatile organic impurities which may be present in the resins.
The presence of excess water and/or volatile organic compounds in the resins can result in problems by causing undesirably high mold pressure and formation of voids or bubbles in the formed composite, resulting in a loss of clarity and strength of the resin composite. Thus, many lower solids content, phenolic thermosetting compositions are unsuitable for liquid injection processes.
Attempts to use high solids content phenolic resins also have been relatively unsuccessful because of the high viscosity of such resins and the presence of voids in the formed composite. The high viscosity of such resins, many of which are solid or nearly solid at room temperature, necessitates high shear mixing with any reinforcing material and injection into the mold under high shear conditions. This can result in significant attrition of the reinforcing material, thus lowering the strength of the final composite. Such premixing also precludes the use of resin transfer molding. The high viscosity of such resins also impedes sufficient wetting of the reinforcing material and adequate mixing with the reinforcing material. The end result is usually products which have poor physical properties, including areas in which there is separation of the resins from the reinforcing materials.
As previously described, phenolic resins are known for their high temperature mechanical strength, low flammability resistance and solvent resistance. Therefore, it would be highly desirable to have phenolic resin composition that do not possess the above described disadvantages and could be used in liquid injection molding methods.